Roundcube

A browser‑based multilingual IMAP client, it delivers a desktop‑like experience within any modern web browser. Users can read, compose, and organize email with full MIME and HTML support, drag‑and‑drop message handling, threaded views, and spell checking. The interface includes an address book with groups, LDAP connectors, and “find‑as‑you‑type” search, as well as multiple sender identities and shared IMAP folders with access‑control lists.

The system runs on a standard web server using PHP and connects to MariaDB, MySQL, PostgreSQL, or SQLite databases. A plugin API and skinning framework allow developers to extend functionality and customize the look and feel, while built‑in caching and support for IDNA, SMTPUTF8, and PGP encryption provide performance and security for unlimited users and messages.

Roundcube is released under GPL‑3.0 with exceptions for plugins and skins, making it free, self‑hostable, and free of subscription requirements. It is maintained as a stable, open‑source project with regular security updates and community contributions.